A conventional fluorescent lamp consists of a sealed tube which has mercury vapor dispersed throughout the tube interior. A phosphor coating is deposited on the inner surface of the lamp tube which faces the tube interior. The lamp tube is formed from a glass which is transmissive of visible light, but is absorptive of ultraviolet (UV) radiation. Operation of the fluorescent lamp is effected by passing an electric current through the interior of the lamp tube, which ionizes the mercury vapor dispersed therein. The ionized mercury vapor emits UV radiation, which is absorbed by the phosphor coating upon contact. The UV radiation fluoresces the phosphor causing the phosphor to emit visible light. The visible light is propagated from the phosphor coating out through the tube to illuminate the surroundings of the fluorescent lamp.
The amount of UV radiation which is converted to visible light is a function of the thickness of the phosphor coating. In particular, the UV radiation conversion efficiency of the fluorescent lamp increases as the thickness of the phosphor coating increases. A thicker phosphor coating provides more active phosphor for the conversion of UV radiation to visible light and also reduces the amount of UV radiation which passes unconverted through the phosphor coating. Unconverted UV radiation passing through the phosphor coating is undesirably lost to absorption by the lamp tube, which reduces UV radiation conversion efficiency.
Although UV radiation conversion efficiency advantageously increases as the thickness of the phosphor coating increases, the light output efficiency of the fluorescent lamp undesirably decreases with increasing thickness of the phosphor coating. A thicker phosphor coating absorbs a larger fraction of the visible light emitted by the phosphor coating before the visible light is able to propagate from the phosphor coating out through the tube. A thicker phosphor coating also absorbs a larger fraction of inwardly propagated visible light which is emitted from the phosphor coating on the opposite side of the tube. Thus, the optimum thickness for the phosphor coating of a conventional fluorescent lamp represents a tradeoff between these two opposing efficiencies, i.e., UV radiation conversion efficiency and light output efficiency. The present invention recognizes a need inter alia for a fluorescent lamp which has improved light output efficiency without diminished UV radiation conversion efficiency or, alternatively, for a fluorescent lamp which has improved UV radiation conversion efficiency without diminished light output efficiency.
Accordingly, it is generally an object of the present invention to provide an illumination device utilizing phosphor for the conversion of fluorescing radiation to visible light, wherein the illumination device has improved light output efficiency without diminished radiation conversion efficiency or, conversely, wherein the illumination device has improved radiation conversion efficiency without diminished light output efficiency. It is another object of the present invention to provide an illumination device utilizing phosphor for the conversion of fluorescing radiation to visible light, wherein the illumination device maximizes the amount of visible light propagated away from the illumination device into the surroundings and minimizes the degree of fluorescing radiation attenuation. It is a further object of the present invention to specifically apply the generalized objectives recited above to the design of fluorescent lamp assemblies. It is still a further object of the present invention to specifically apply the generalized objectives recited above to the design of flat panel display backlight assemblies. It is another object of the present invention to specifically apply the generalized objectives recited above to the design of light-emitting diode (LED) assemblies. It is yet another object of the present invention to specifically apply the generalized objectives recited above to the design of aperture fluorescent lamp assemblies.
These objects and others are accomplished in accordance with the invention described hereafter.